So part of the thing about the Taiping rebellion is that more than just sheer combat culminated in the death toll. Far bigger contributors were famine and disease. For instance, both the rebels and the Qing would loot and burn the fields of farmers in areas to deny the food supplies to the opposing side, resulting in mass famine. The sheer amount of economic damage to China alone is estimated (from a GDP estimate basis) to have taken something like 40-50 years to recover. Considering that it was essentially concentrated in the very populated Central Plains area, it is hardly surprising that so many people ended up dying. A better contrast from WWI would be, say, the 30 Years War, where much of Germany was effectively burned to the ground by various armies, wiping out something like 1/3rd of the population.